A hip joint prosthesis includes an acetabular component and a femoral component. The acetabular component often includes a shell (or maybe cup) part, which is implanted in the patient's acetabulum, and a liner. The femoral component includes a stem part which fits into the intramedullary canal of the patient's femur. The stem part has a neck at its proximal end. A head part is fitted on to the neck. The convex surface of the head part provides a femoral component bearing surface which acts against the corresponding bearing surface in the acetabular component provided by concave internal surface of the liner.
The liner can allow smooth movement of the head part within the acetabular cup. During surgery, the liner is inserted into the cup after the cup has been installed in the acetabulum of the patient. It is important that when the liner is inserted into the cup, the liner is aligned correctly with respect to the cup. The cup and the liner are usually substantially hemispherical. For correct alignment, it is generally required that the pole of the substantially hemispherical liner coincides with the pole of the substantially hemispherical cup. In this configuration, the rims of the cup and the liner are generally also in alignment (e.g. contained within a common plane).
Misalignment of the liner with respect to the acetabular cup can lead to a number of problems. If the misalignment is noticed during surgery, the surgical procedure may need to be extended to correct the problem. If the misalignment is not noticed, it can lead to restricted movement of the hip joint, component loosening, component damage (e.g. cracking) and potentially the need for further surgery to correct the problem.
Devices are available that assist in inserting the liner into the acetabular cup. These devices are typically mounted on the liner itself prior to positioning the liner over the cup for insertion. Some devices of this kind allow for initial alignment of the liner with respect to the cup prior to insertion. However, these devices are prone to tilting of the liner as it is inserted into the cup, leading to misalignment.
WO 2011/161166 describes an insertion instrument for instrumented insertion of a socket inset with a spherical cap into a hip socket of a hip-joint prosthesis, having an impacting instrument with a handle, at one end of which a holding tool for the socket inset is located.
WO 2008/106598 describes an acetabular liner insertion guide that aligns a liner within an acetabular shell. The liner includes a ring and a penetrable layer. The ring includes a lip configured to rest on an upper surface of the acetabular shell. The ring is also configured to attach to the liner such that an upper surface of the liner is in a plane that is generally parallel to a plane that includes the upper surface of the acetabular shell. The penetrable layer is configured to receive an impactor and overlie the liner. When the insertion guide is placed on the shell and the impactor impacts the liner, the insertion guide separates from the liner and remains on the impactor.